February 16, 2005
02:00 PM (EST)

News Release Number: STScI-2005-06

Saturn's Auroras Defy Scientists' Expectations

The full news release story:

The dancing light of the auroras on Saturn behaves in ways different
from how scientists have thought possible for the last 25 years. New
research by a team of astronomers led by John Clarke of Boston
University has overturned theories about how Saturn's magnetic field
behaves and how its auroras are generated. Their results will be
published in the February 17 issue of the journal Nature.

By choreographing the instruments aboard the Earth-orbiting Hubble Space
Telescope and the Cassini spacecraft, while it was enroute to Saturn, to
look at Saturn's southern polar region, Clarke and his team found that
the planet's auroras, long thought of as a cross between those of Earth
and Jupiter, are fundamentally unlike those observed on either of the
other two planets. The ruby-colored lights that occasionally paint the
sky over Saturn may, in fact, be a phenomenon unique within our solar
system.

In Clarke's experiment, Hubble snapped ultraviolet pictures of Saturn's
auroras over several weeks and Cassini recorded radio emissions from the
same regions while measuring the solar wind, a stream of charged
particles that trigger auroras. Those sets of measurements were combined
to yield the most accurate glimpse yet of Saturn's auroras.

The observations showed that Saturn's auroras differ in character from
day to day, as they do on Earth, moving around on some days and remaining
stationary on others. But compared with Earth, where auroras last only
about 10 minutes, Saturn's auroras can last for days.

The observations also indicated, surprisingly, that the sun's magnetic
field and solar wind may play a much larger role in Saturn's aurora than
previously suspected. Hubble images, when combined with Cassini
measurements of the solar wind, show that it is the pressure of the solar
wind that appears to drive auroral storms on Saturn. In Earth's case, it
is mainly the sun's magnetic field, carried in the solar wind, that drives
auroral storms.

Seen from space, an aurora appears as a ring of light circling a
planet's polar region, where magnetic poles typically reside. Auroral
displays are initiated when charged particles in space collide with a
planet's magnetic field and stream into the upper atmosphere. Collisions
with gases in the planet's atmosphere produce flashes of glowing energy
in the form of light and radio waves.

Scientists had long believed Saturn's auroras possess properties akin to
both Earth and Jupiter. Like Earth's, they were thought to be influenced
by the solar wind. Like Jupiter's, they were assumed to be influenced by
a ring of ions and charged particles encircling the planet. The new results
do show, however, a feature of Saturn's aurora that matches Earth's: Radio
waves appear to be tied to the brightest auroral spots. This similarity
suggests that the physical processes that generate these radio waves is
just like those of Earth.

But, as the team observed, though Saturn's auroras do share
characteristics with the other planets, they are fundamentally unlike
those on either Earth or Jupiter. When Saturn's auroras become brighter
(and thus more powerful), the ring of energy encircling the pole shrinks
in diameter. When Earth's auroras become brighter, the polar region for
several minutes is filled with light. Then the ring of light dims and
begins to expand. Jupiter's auroras, however, are only weakly influenced
by the solar wind, becoming brighter about once a month, at the most, in
response to solar wind changes.

Saturn's auroral displays also become brighter on the sector of the
planet where night turns to day as the storms increase in intensity,
unlike either of the other two planets. At certain times, Saturn's
auroral ring was more like a spiral, its ends not connected as the
energy storm circled the pole.

Now that Cassini has entered orbit around Saturn, Clarke and his team
will be able to take a more direct look at the how the planet's auroras
are generated. According to Clarke, the team will next probe how the
sun's magnetic field may fuel Saturn's auroras and what role the solar
wind may play.